Selector switch circuit



Dec. 14, 1943. c. R. GRAY 2,336,861

SELECTOR SWITCH CIRCUIT Filed Sept. 19, 1942 H6.

M/VENTOR C. R. GRAY ATTORNEYv Patented Dec. 14, 1943 UNITED STATES PATENT ()FFICE SELECTOR SVJITCH CIRCUIT Cecil R. Gray, Morristown, N. .l., assignor to Bell Telephone Laboratories, Incorporated, New

2 Claims.

This invention relates to circuit arrangements of machine switching devices particularly those used in communication systems.

The object of the invention is to provide means to eliminate a certain infrequently occurring interaction between two selector switches which results in double connections.

In the operation of the well-known step-bystep selector switch there is what is spoken of as an unguarded interval between the time the brushes of the selector are placed on the terminals of an idle line and the time the selector seizes such idle line. It is necessary to have such an unguarded interval in order to test for the busy or idle condition of the line, but it is likewise desirable to make this interval as short as possible to prevent the seizure of the line by another selector.

The circuits of a well-known type or" selector are such that as the test brush of the selector reaches an idle line it has a ground connection thereto which however is almost immediately opened to allow the test to be made. If at this instant another selector should reach a multiple appearance of this line the ground which it first applies to the line will extend back over the multiple connection so as to make the line appear busy to the first selector. This first selector will then take another step and in doing so will apply ground over the multiple until the step is actually taken to make the line appear busy to the second selector. Hence the second selector also takes another step. This action is known as chasing and may continue for several steps during which time the two selectors tend to become synchronized with each other with the result that eventually the unguarded interval of both will occur at the same instant and the line appearing idle to both selectors, both will seize it and a double connection will be established.

In accordance with the present invention a unidirectional resistance is placed in the testing circuit of the selector so that the ground brought to the terminal of an idle line cannot be communicated over the multiple connection during the unguarded interval of and to the test relay of another selector which has previously reached such line.

A feature of the present invention is the use of a varistcr or unidirectional resistance in the test circuit of a selector switch to prevent the connection of a low resistance ground to the test wire of a line which might otherwise be communicated over a multiple connection and cause an instandard operation of another selector which has reached the same line.

Other features will appear hereinafter.

The drawing consists of a single sheet having four figures as follows:

Fig. 1 is a circuit diagram of a pair of selector switches both placed in cooperative relationship with the same selected line, the upper one being shown in full and the lower one being shown only in part;

Fig. 2 is a schematic diagram showing the essential selector circuits of two selectors in which the means of the present invention is not incorporated at the instant that the first comes into cooperative relationship with a line to which the second has connected and while the second is in its unguarded interval;

Fig. 3 is a similar schematic showing the circult condition an instant later as the first of the selectors goes into its unguarded interval and as the second of the selectors is impelled to move forward as the result of a false busy condition found under the conditions of Fig. 2; and

Fig. 4 is a similar schematic showing the circuit condition a slight interval thereafter as the first of the selectors returns a false busy condition and as both then prepare to step away from an idle line to the next terminal, practically simultaneously.

The operation of the selector may be understood lay the following description of the upper selector circuit of Fig. 1. The selector is seized by the movement of the tip, ring and sleeve brushes l, 2 and 3, respectively, engaging the terminals 4, 5 and 5. A circuit is then established from ground, secondary winding of transformer l acting as a source of dial tone, springs 2 and I of the eleventh rotary position springs 8, upper winding of the A relay 9, back contact and armature I of D relay it, tip contact A, tip brush I, to the loop of the line which has controlled the brushes I, 2 and 3, ring brush 2, ring contact 5, armature l and back contact of D relay Iii, lower winding of A relay 9 to battery. Relay 9 responds and causes the energization of the first slow B relay il in a circuit from ground, armature 2 and back contact of D relay I 0, armature and front contact of A relay 9, winding of B relay l l to battery. Relay H responds and prepares a circuit for the second slow C relay l2 and the vertical magnet I3.

When dial pulses are transmited from the loop holding the A relay 9, each pulse takes the form of a short interruption of the circuit of A relay 9 and this relay responds by releasing its armature for a short interval. During this interval a circuit is established from ground, armature 2 and back contact of D relay it, armature and back contact of A relay 9, armature l and front contact of B relay ll, winding of C relay l2 and the winding of the vertical magnet lit to battery whereupon the brushes it, it and it are stepped in a vertical direction.

On the first vertical step the vertical off-normal contacts I"! are operated and due to the operation of C relay l2 the circuits for the release magnet l8 and the rotary magnet is are prepared. A circuit may be traced from ground, armature 2 and front contact of B relay H, front contact and armature of C relay i2, springs 13 and 2 of the vertical off-normal springs ll, winding of E relay 2% to battery. E relay it responds and prepares a circuit for the rotary magnet it which'will become efiective on the release of C relay E2. The D relay it is now short-circuited from ground on its own armature 2, through armature i and rotary magnet. Just as the E relay and the rotary magnet it releases a point leading in one direction through tion through the hack contact and rotary magnet 59, springs i and 2 off-normal springs ll, winding of battery. During this, the unguard d while the sleeve brush is on the tern outgoing line and before ground is placed thereon by this selector switch a test is oi the selected line. If the line is busy ground will be on the sleeve terminal with a consequence that D relay iii will be short-circuited and relay is will become energized. This will result in another energization of the rotary stepping magnet. 11" the line is idle no ground will be connected to the junction point between the D and rel ys. Thereupon the D relay it will inc energized in series with the E relay The resist noes oi these two relays are such that the D relay will energize but the E relay will remain released.

again energized.

It is to be noted that the varistor 2i a devic which presents a low resistance to current 1 ground found by sleeve brush it but will or a high resistance to curr ture 2 and back contact and front contact 55%, W1 closed as the brush mo /es on to of the selected line.

As a part of Fi another selector is unguarded interval so that the junctic bai /19311 relays 22 and 23 isconnectcd to the atthis instant the crush 5-" ould to a terminal multipled to the terminal on which part its .brushzt isresting a circuit will be extended from ground, armature 2 and back contact of D relay iii, armature l and front contact of E relay 2%], varistor 2% (high resistance direction) back contact and armature t of D relay it, brush l6 through the multiple terminals to brush 2s, armature 3 and back contact of D relay 22, varistor 25 (low resistance direction) to the test circuit of the skeietonized selector. Due to the varistor 2! the ground placed on the multiple sleeve connection at this instant is of such high resistance that the test being made by the skeletom'zed selector is carried out without interference by the selector which has just arrived.

Figs. 2, 3 and 4 will explain the trouble that occurs when the varistors 2i and 25 are not included in the circuit. Fig. 2 shows-th condition just described, with the lower of the two selectors in its unguarded interval and the upper of the two selectors just arrivin on the multipled terminal. Thus the lower of the two selectors will apparently find a busy line, whereupon the E relay 23 will respond. ihis is indicated in Fig. 3 which represents the condition established immediately afterwards. Now as the upper of the two selectors goes into it un uarded interval it will find a ground returned through the armature of relay which just been energized .poer selector as in Fig. 2. Therefore, the concnuon of his. 4 will he established wherein each selector has tested a busy condition. This action te ds to synchronize the two selectors after seve 1 steps so that if th re are enough idle ter is before them they will eventually both go ito their unguarded intervals at the same time and a double connection will be established. c are not enough idle terminals to allow --.s synchronization to become effective then one will chase the other to the eleventh rotary step and a busy condition w' l he reported. of these results are undesirable as both nt failures to find an idle line.

g the varistors in the circuit as shown selector is prevented from placing a g u lad previously arrived and had gone into its guarded terval. Thus, no select-or is driven a unid; resistance element and a back contact and an armature of said relay D to said "test wiper, said unidirectional resistance element being poled to present a high resistance in the direction from series circuit to said test arrang ment for a selector switch g false operation or" a litre switch 0 a connection, comprising a test 'th the multiple bank "*h the winding of said relay to a circui junction, through the winding of said relay D to ground and from said circuit junction through a unidirectional resistance element, a back contact and armature vofsaid relay D to said test wiper, said relay E being responsive to ground encountered by said test wiper and said relay D operating in series with the winding of said relay E without operating said relay E responsive to the failure of said test wiper to encounter a ground connection, a locking ground circuit for said relay E including an armature and front contact of said relay E, said unidirectional resistance element being poled to present a low resistance value to current flowing from a ground encountered by said test wiper and a high resistance value to current flowing from said locking ground for said relay E to said test wiper. CECIL R. GRAY. 

